Since the 1940s, it has been generally accepted in the medical community that the growth of prostate cancer is androgen-dependent,3 and prostate cancer risk is the primary safety concern with use of exogenous testosterone.4, 5 Prescription testosterone has been available in the U.S. for more than 50 years, but patterns of uptake have recently changed. U.S. sales of testosterone have increased more than 1500% considering 1994-2002, with sharp increases in the most recent years.6 Researchers have noted that the increase in uptake is occurring in the absence of changes in recommendations for testosterone use,7 e.g., an expansion of approved indications. There is very little information regarding the characteristics of users, and clinical trials have not provided long-term safety data regarding the prostate cancer risk of testosterone products.5, 6 There have been no prior epidemiologic studies of exogenous testosterone products and prostate cancer risk, possibly because of the large number of men needed to capture sufficient numbers exposed to testosterone. The research intent of this application is to cost-efficiently use a unique existing resource, the Saskatchewan Health Services Databases, in order to fill the knowledge gap surrounding exogenous testosterone use and prostate cancer and deliver the first population-based epidemiologic study on this topic. The provincial government of Saskatchewan provides health coverage to nearly all residents, and the existing data contain >500,000 males and >30 years of observation time from a defined geographic area, with cancer outcome information available from a provincial cancer registry.57, 58, 105 Testosterone exposures are captured in electronic prescribing records and are not subject to error based on patient recall, with dose and duration of use available. We plan to use a retrospective, matched cohort study design and longitudinal analysis methods to estimate the risk of prostate cancer incidence and recurrence, incidence of severe prostate cancer, prostate cancer mortality, and breast cancer incidence among males exposed to exogenous testosterone compared to age- matched males not exposed. We will also descriptively examine patterns of testosterone use to better understand the characteristics of users. The diverse expertise of the research team includes pharmacoepidemiology, biostatistics, clinical endocrinology, and oncology, and the team has a demonstrated history of manuscript productivity relevant to the publication plans of the project. The public health relevance of the proposal is high, given the increasing using of testosterone as 'hormone replacement therapy'and the potential for newly repeating 'the estrogen disaster'among women for prostate cancer among men.16 The clinical implications for the patient are a cessation of testosterone therapy, if increased risks are found. Regardless of the direction of results, our findings will serve the federal health goals of pharmacovigilance for marketed medications.17 PUBLIC HEALTH RELEVANCE: This study considers whether men who take prescription testosterone might be more likely to later develop prostate cancer and breast cancer. The large study looks at men who used testosterone from 1977-2007 in the province of Saskatchewan, Canada, and then looks later in time to see if they have higher rates of prostate cancer compared to men who never used testosterone. The results of the study might help determine if testosterone is safe for men to use today.